The objective of this research program is to construct a consistent and coherent account of the development of scientific reasoning. Among the areas that will be studied are children's understanding of physical scientific concepts, biological concepts, and mathematics. Concepts such as time, proportionality, and life and procedures such as counting, numerical magnitude comparison, and addition will be among those given the most attention. Investigation of these concepts and procedures will focus on children's existing knowledge, their ability to acquire new knowledge, and the basic processes that may underlie developmental changes in the first two domains. These issues will be explored with children of a wide span of ages, ranging from three years through adolescence. Past research indicates that many of the scientific concepts under investigation have a desirable property for developmental research; even very young children know something about the tasks, while even adults do not know everything there is to know. Thus, the problems will allow us to observe development over an unusually protracted time period. A variety of theoretical perspectives will be brought to bear on the issues described above. The proposed experiments are guided by ideas derived from Piagetian, information processing, and traditional learning models. All of these theories have contributed both to the content of the present approach and to the methodologies that will be used.